Recently, mobile telephone services were introduced wherein a subscriber only needs one single mobile phone that is suitable for calling via VOIP and via GSM/UMTS. If the subscriber resides at home, she may call via a radio based connection like WLAN, WIFI, Bluetooth. These radio based connections are collectively denoted as UMA (Unlicensed Mobile Access). GSM, UMTS and the like are denoted as GAN (Global Access Networks). Phones capable of connection in both UMA and GAN are denoted as UMA/GAN phones. For a subscriber to use the phones capabilities she needs to have either GAN and UMA subscriptions or a combined UMA/GAN subscription.
If an UMA/GAN subscriber is out of reach of an UMA area, she will call via the GSM/UMTS network. In order to call the subscriber, a mobile telephone number has to be dialed which is most of the time more expensive than dialing to a fixed number.
Most people still use their fixed telephone numbers next to their mobile telephone. If they are called at their fixed number, they may forward the calls to their mobile number using a rerouting service. If they dial a prefix, such as for example dialing *21, and then the mobile number, all calls to the fixed number are forwarded to the corresponding mobile phone. In this way, they will always be reached. Part of the cost for the forwarded calls may however be billed to the called party.